The main function of vehicle suspension is to dampen and reduce dynamical loads (e.g. vibrations) transferred to vehicle body by vehicle wheels. To do its work in the optimal way vehicle suspension should have suitably variable suspension rate and thus nonlinear (progressive) damping characteristic, capable of being adjusted to vehicle weight and dynamical loads.
Suspensions of automotive vehicles are fitted with steel springs such as leaf springs, coil springs, torsion bars, as well as solid rubber elements and pneumatic springs and hydropneumatic elements.
All the types of steel springs alluded to above feature, in principle, constant spring rate and thus linear characteristic. Vehicle suspensions fitted with steel springs are compact, robust, durable and reliable, but they usually feature constant suspension rate and linear damping characteristic inherited from the linear characteristic of the steel spring they use, which is considerably inferior to that of the pneumatic suspension. Vehicle suspensions with steel springs can be made progressive e.g. by applying several in turn actuating springs; then a non-differentiable progressive damping characteristic of the suspension is being obtained. Some of them, e.g. those using coil springs, can be made progressive without disturbing the differentiability of their characteristic. However in both these cases the suspension damping characteristic still remains substantially inferior to that of the pneumatic and hydro-pneumatic ones.
Pneumatic and hydro-pneumatic vehicle suspensions feature favorable differentiable progressive damping characteristic, but they are expensive and much less durable and reliable than suspensions fitted with steel springs. Moreover, a major disadvantage of both the suspensions in question is that their damping characteristic is determined by the thermodynamic parameters of the gas (air or nitrogen) they utilize (namely the adiabatic exponential), and therefore cannot be freely adjusted to specific requirements. Moreover, the damping characteristic of such suspensions is still far from optimal.
A common feature of all commonly used vehicle suspension systems is that their characteristic is determined in great part by the characteristic of the spring being used, and therefore is hard to make optimum.
Some unconventional vehicle suspension systems providing non-linear damping characteristic and means for adjusting it are known from prior art. For example the International Publication WO-A-96 11815 of the International Application PCT/CA 95/00570 discloses a suspension system, in which the suspension arm rotates roller carriers, the rollers contained therein follows cam surfaces, which in turn force a spring supports to move axially and to compress the spring. The U.S. Pat. No. 3,157,394 granted to Mr. O. K. Kelley in 1964 provides another example of suspension with a cam mechanism, a number of in turn actuated Belleville springs and non-linear non-differentiable damping characteristic. However nonlinearity of the damping characteristic of these suspensions is achieved by engaging springs through a cam mechanism, and means for adjusting the characteristic are shape of the cam, its position relative other elements of the suspension mechanism and nuts to regulate the initial length of the spring. Consequently, these suspensions are excessively complicated, of questionable, if not doubtful, durability and reliability, unable to cope with large loads, and means for adjusting damping characteristic of them are completely unsatisfactory. Another example of variable rate vehicle suspension is provided by the U.S. Pat. No. 4,010,941 granted to Mr. A. C Kirkland in 1977. In this example variability of spring rate of torsion bars, and thus variability of suspension rate of vehicle suspension fitted with torsion bars, is attained by providing means to vary torsion bar's length. Again this suspension is far from being satisfactory for at least the following reasons: For the first, to adjust (increase) the torsion bar spring rate to increasing vehicle weight, the length of said torsion bar is being decreased, which diminishes the spring capability of accumulating potential energy. For the second, means for adjusting suspension rate can by applied exclusively to suspensions using torsion bars. For the third, means for adjusting suspension rate are not capable of varying the spring rate continuously, and therefore the damping characteristic of the suspension is not differentiable and far from optimal.
Thus there is a need for a vehicle suspension combining and even exceeding the advantages of both the steel and hydro-pneumatic suspensions, having differentiable non-linear damping characteristic capable of being freely adjusted to any specific requirements.